1. Field of the Invention
The present invention relates to a scanning type exposure apparatus and method, and a device manufacturing method, wherein the exposure apparatus is preferably used in a lithography process for manufacturing micro-devices.
2. Description of the Related Art
In a lithography process of micro-device (semiconductor) manufacturing, two types of exposure apparatuses are known. One is a step-and-repeat type (so-called a stepper) and the other is a step-and-scan type (so-called a scanner or a scanning type exposure apparatus).
The former type uses a step-and-repeat sequence for transferring stepwise a pattern of an original (e.g., a reticle or a mask) onto a substrate (e.g., a semiconductor wafer or a glass substrate). In that sequence, the whole pattern of the original is illuminated and is projected onto one of a plurality of exposure regions of the substrate through a projection optical system, while the original and the substrate are maintained stationary. Next, the substrate is moved stepwise to change the exposure region and then exposure is repeated in the same manner.
On the other hand, the latter type uses a step-and-scan sequence. In that sequence, the original pattern is illuminated with a slit-like beam and a portion of the pattern is projected onto one of the exposure regions of the substrate, while both the original and the substrate are scanningly moved. Then, the substrate is moved stepwise to change the exposure region, and thereafter the exposure is repeated in the same manner.
The latter step-and-scan type exposure apparatus seems to have become the mainstream, expected at least in the near future, for the reason that it has a high potential of exposure performance in terms of an increase in transferring precision and field size.
The scanning type exposure apparatus mainly comprises stage devices (e.g., a wafer stage or a reticle stage) for scanningly moving the wafer and the reticle (mask), an illumination optical system and a projection optical system. In this apparatus, while scanning exposure is performed, the wafer stage and the reticle stage are synchronously controlled by a master-slave control method, in which the wafer stage is a master stage and the reticle stage is a slave stage, and the reticle stage follows the movement of the wafer stage in the scanning direction. The master-slave control method provides an advantage in improving movement resolution, especially when the projection optical system is a reduced scale system.
The apparatus also includes detection systems, e.g., laser interferometers for detecting the stage positions in the scanning direction, and a focus detector for detecting a distance between the wafer surface and a focal plane of a projection optical system during the scanning exposure, for adjusting the wafer surface with respect to a focal plane of the projection optical system.
Time series outputs from the focus detector tend to have smooth variations. With the recent advancements in wafer planarization processes and to increase pattern transfer accuracy, wafers are processed to be as smooth as possible. As a result, the focus detection outputs typically do not have sudden variations, at least within each chip area (shot area).
Conversely, adjusting the wafer surface in the tilt direction according to a detection having sudden variations causes defocusing within the chip area. For example, if a 50 ppm inclination was obtained as the focus detection on a condition that the chip area has a 20 mm size in a longitudinal slit direction, there exists 50 ppm×20 mm=1 micron of height difference in a Z direction between edge portions of the chip area. Since the recent exposure apparatuses have a DOF (Depth Of Focus) of at most 0.7 micron, it is difficult to perform exposure without causing any defocusing in each of the chip areas.
One of the expected causes of sudden focus detection variations is the presence of contaminated particles on the wafer or the wafer chuck surface. For example, wafer resist exfoliation particles are apt to cling onto the wafer surface or the chuck surface in the form of such contaminated particles. Semiconductor manufacturing processes pay close attention to reduce such resist exfoliation particles, and regular maintenance checks are performed to replace and clean the wafer chuck. However, the contaminated particles on the chuck are frequently not detected. Therefore, the chuck may become contaminated during the term between the maintenance checks. Consequently, productivity (e.g., chip production yield) deteriorates.